Ernst Mach was born in 1838 in Chrlice (now part of Brno), when
Czechia was still a part of the Austro-Hungary empire.
Johann, Mach's father, was a high school teacher who taught Ernst at
home until he was 14, when he studied in Kromeriz Gymnasium, before
he entered the university of Vienna were he studies mathematics,
physics and philosophy.
He graduated from Vienna in 1860.
There Mach wrote his thesis "On Electrical Discharge and Induction."
Mach was interested also in physiology of sensory perception.

Figure 1.6:

Photo of Ernest Mach

At first he received a professorship position at Graz in mathematics
(1864) and was then offered a position as a professor of surgery
at the university of Salzburg, but he declined.
He then turned to physics, and in 1867 he received a position in
the Technical University in Prague1.48
where he taught experimental physics for the next 28 years.

Mach was also a great thinker/philosopher and influenced
the theory of relativity dealing with frame of reference.
In 1863, Ernest Mach (1836 - 1916) published Die
Machanik in which he formalized this argument.
Later, Einstein was greatly influenced by it, and in 1918,
he named it Mach's Principle.
This was one of the primary sources of inspiration for Einstein's
theory of General Relativity.

Figure:The Photo of the bullet in a supersonic flow that Mach
made. Note it was not taken in a wind tunnel

Mach's revolutionary experiment demonstrated the existence of the
shock wave as shown in Figure 1.7.
It is amazing that Mach was able to photograph the phenomenon using
the spinning arm technique (no wind tunnel was available at that time
and most definitely nothing that could take a photo at supersonic
speeds.
His experiments required exact timing.
He was not able to attach the camera to the arm and utilize the
remote control (not existent at that time).
Mach's shadowgraph technique and a related method called
Schlieren Photography are still used today.

Yet, Mach's contributions to supersonic flow were not limited to
experimental methods alone.
Mach understood the basic characteristics of external supersonic flow
where the most important variable affecting the flow
is the ratio of the speed of the flow1.49 (U) relative to
the speed of sound (c).
Mach was the first to note the transition that occurs
when the ratio U/c goes from being less than 1 to greater than 1.
The name Mach Number (M) was coined by J. Ackeret (Prandtl's
student) in 1932 in honor of Mach.